Disk drive devices using various kinds of disks, such as an optical disk, a magneto-optical disk, and a flexible magnetic disk, have been known in the art. In particular, a hard disk drive (HDD) has been widely used as a storage device of a computer and has been one of indispensable disk drive devices for current computer systems. Moreover, the HDD has found widespread application such as a removable memory used in a moving image recording/reproducing device, a car navigation system, a cellular phone, or a digital camera, as well as the computer due to its outstanding characteristics.
Magnetic disks used in the HDD have data tracks formed concentrically and servo data containing address information, and on each data track a plurality of data sectors containing user data are recorded. Between each servo data, data sectors are recorded. A head element portion of a head slider supported by a swinging actuator accesses a desired data sector according to address information of servo data so that writing and reading to and from data sector can be performed.
In order to improve recording density of the magnetic disk, it is important to reduce the clearance between the head element portion flying over the magnetic disk and the magnetic disk. Hence, some mechanisms have been proposed to adjust the clearance. In one of them, a head slider is equipped with a heater which adds heat to the head element portion to adjust the clearance (refer to Japanese Patent Publication No. 2006-190454, for example). In the present specification, this is referred to as thermal flyheight control (TFC). The TFC supplies the heater with electric current to generate heat so that the thermal expansion makes the head element portion protrude. Thereby, the clearance between the magnetic disk and the head element portion is reduced. Mechanisms such as using a piezo element to adjust a clearance are known other than TFC.
The clearance varies in response to the altitude (atmospheric pressure) change in addition to temperature change (refer to Japanese Patent Publication No. 2006-092709, for example). Therefore, it is preferable to detect the altitude change in addition to the temperature change to adjust the clearance accordingly.
Typical TFC increases the heater power according to the temperature decrease to make the head element portion protrude and compensates an increase in the clearance due to the temperature decrease. The clearance between the head element portion and the magnetic disk reduces accordingly. Therefore, the TFC in accordance with the altitude change requires a scheme different from the ordinary TFC in accordance with the temperature change.
An altitude sensor (atmospheric pressure sensor) has been known as one way of detecting altitude. Use of the altitude sensor, however, results in an increase in the number of parts and the cost. The clearance between the head element portion and the magnetic disk can be determined with signal strength read out by the head element portion. Specifically, the signal strength increases with a decrease in the clearance. Thus, measuring the signal strength allows adjusting the clearance so as to compensate its changes without an altitude sensor.
It is known, however, that the read-out characteristics of the head element portion changes significantly with time of use. Here, this characteristics change is referred as instability. The read-out signal strength of the head element portion changes with the characteristics change of the head element portion. Therefore, a technique for handling the instability is required in order to determine the clearance change from the read-out signal strength.
Determination of the altitude (atmospheric pressure) is also important from another point of view. For example, measuring the clearance between the slider and the magnetic disk is as important as measuring the clearance between the head element portion and the magnetic disk. It enables to avoid a collision of the slider and the magnetic disk. In such a case, it is important to determine the altitude more precisely. Thus, it is important to eliminate or reduce the influence of the instability in determining the altitude with the read-out signal.